The brain is vigilant in control of breathing, regulating blood O2 and CO3 over an order of magnitude range in metabolic demand, wide ranges of posture and body movements, compromises in muscle or cardiopulmonary function, from birth till death without lapses beyond a few minutes. This grant continues our effort to determine the sites and mechanisms underlying the nervous system generation and modulation of breathing rhythm. Determination of these properties is critical for understanding pathologies where ventilatory failure results from dysfunction of nervous system, including such diseases as apnea of prematurity, congenital central hypoventilation, central alveolar hypoventilation, sleep apnea and, perhaps, sudden infant death syndrome. Significant progress in identifying critical sites and cellular mechanisms involved in the generation of the respiratory rhythm followed the development of novel in vitro preparations. These preparations generate a respiratory-related rhythm, and their exploitation led to two hypotheses that are the focus of this application: SITE HYPOTHESIS: The preBotzinger Complex (preBotC) contains the kernel for respiratory rhythm generation. The preBotC is the limited portion of the ventrolateral respiratory column necessary and sufficient for in vitro neonatal rodent medullary slice preparations to generate respiratory-related motor nerve output. We discovered that the anatomical extent of the preBotC in rodent is demarked by neurons expressing Neurokinin 1 receptors. Two goals of this project are to use this information to: i) test the SITE HYPOTHESIS, and; ii) identify the preBotC in neonatal and adult human brain. RHYTHMOGENESIS HYPOTHESIS: Endogenous bursting, i.e., pacemaker(-like), neurons in the preBotC are the cellular kernel for respiratory rhythm. There are three questions that must be answered to understand the generation of the respiratory rhythm: i) Which neurons are necessary and/or sufficient for the rhythm?; ii) By what mechanism do these neurons generate the rhythm?; How is the rhythm modulated? Of the various neurons which are candidates for this role, we propose a population of neurons called Type-1. Two additional goals are to: ) test whether Type- 1 neurons are required for generation of respiratory rhythm, and; ii) determine the synaptic interactions amongst preBotC neurons that define the respiratory rhythm generating network.